BRUSSELS—Authorities identified two suicide bombers who hit the airport and subway here as brothers with criminal pasts, and began to trace a clear line between their plot and the terrorists behind the November killings in Paris.

The Belgian capital was on high alert a day after at least 31 people were killed and more than 300 wounded in the coordinated attacks, which came just days after investigators in Brussels thought they had made a breakthrough in their fight against a violent Islamic State cell by capturing the last...

RELATED VIDEO

Brussels Attacks: What Clues Did the Explosives Reveal?

University of Rhode Island chemistry professor and explosives expert Jimmie Oxley on the kinds of bombs, bomb elements terrorists are believed to have, what clues damage can provide. Photo: Getty